T cell lymphomas and B cell lymphomas have been induced in C57BL/6 mice by different replication-able recombinant retroviruses (mink cell focus-inducing (MCF) viruses), isolated from x-irradiated mice. The structure of the cloned recombinant viruses and associated mouse-tropic and xenotropic viruses is studied by restriction mapping of nonintegrated viral DNA and of x-ray-induced and virus-induced tumor cell DNA's. Analysis of the respective viral gene products is done by immunoprecipitation of infected and transformed cell extracts using type-specific antisera made against cell surface-associated viral antigens. Tumor RNA of (virus-free) x-ray-induced lymphomas will also be analyzed by Northern blotting of tumor cell RNA, using specific nick-translated DNA probes.